It has. There is a great video we watching in one of my first physics classes. They have two atomic clocks (Cesium). One on earth and one on a very high altitude plane traveling very fast. Once the plane landed, they compared the clocks and found a distinct difference in the time. I forget how much time, but it was there.

Originally posted by einsteinian77 Has time dilation ever been proven experimentally?

Well no theory can ever be proven. However it's been confirmed quite adequately. Buy that I mean that we use a theory to make a prediction. We then do an experiment - Are the results of the experiment consistent with the prediction? If so then we're more comfortable with the prediction

For instance - the depth of penetration of muons into our atmosphere is consistent with relativity. Muons decay and have a particular half life. We can use that to see if this half life changes with speed - it does - therefore ther results are consistent with the theory.

Originally posted by PhysicsRocks88 No theory can ever be proven? Could a more stupid statement come from an animals mouth?

You need to calm down. Such displays of hostility will generally not be tolerated here.

Futher, pmb is exactly correct -- no physical theory can ever be proven.

You cannot ever prove a physical theory, because to do so would require that you demonstrate that it works in every possible experiment -- and there are an infinite number of possible experiments. No matter how many experiments you do which all agree with your theory, there will be more experiments left to do -- and you cannot be sure they won't disagree with your theory.

The best you can hope to do in science is to declare that your theory is "proven" within the domain of the experiments you've subjected it to. You cannot say anything about its applicability to experiments that have not yet been performed.

Originally posted by PhysicsRocks88 WHAT? Who in the hell on earth are you?

No theory can ever be proven? Could a more stupid statement come from an animals mouth?

I suggest you take that back.

Yep. It's true. It's always concievable that someone may make an observation which contradicts the theory. So you just keep testing it. The greater the number of predictions tested the more confidence one has in a theory. But its a well accepted fact in all branchs of science that no theory can be proved correct. They can at most be proved incorrect.

The slowdown of the atomic clock in the 'airplane-experiment' could well be explained by other than relativistic effects, for instance by the Lorentz- Force in the earth's magnetic field B: for typical aircraft speeds, the potential energy change in an atom due to the Lorentz Force (W=r*e*B*v/c (in Gaussian cgs-units with r= Bohr Radius; e= elementary charge)) is about a factor 10^-7 smaller than the potential energy of a quantum mechanical oscillator (W=h*v) of a typical atomic clock (v=1 GHz (=10^9 Hz)). As the corresponding atomic transitions are spin- rather than orbit related, this is a second order effect. i.e. the frequency shift depends on the square of the electric potential. This yields therefore a similar order of magnitude for the resulting frequency shift as the relative time dilation suggested by Relativity (i.e. about 10^-13) and also the same velocity dependence ~(v/c)^2 (if v small compared to c). For a better estimate of this effect one would have to solve the Schrödinger Equation for the disturbed potential however. Other possible causes of a clock slowdown could be due to mechanical (i.e. non-gravitational) accelerations and/or vibrations which should induce disturbances in the inner-atomic fields and therefore a corresponding frequency change (again a quantum mechanical calculation might be necessary to confirm this quantitatively).

By the way, as far as I am aware, by no means all experimental data show clear evidence for the existence of time dilation both in Special and General Relativity, and those that are usually cited in support of this effect often have substantial systematic and/or statistical uncertainties.
Unfortunately, this counter- evidence is mostly suppressed (habitually if not systematically) as it obviously does not fit into the official view. But, as indicated above, 'experimental evidence' is anyhow irrelevant for a matter of conceptual consistency (see my pages http://www.physicsmyths.org.uk/relativity.htm and http://www.physicsmyths.org.uk/lightspeed.htm).

Originally posted by Thomas
By the way, as far as I am aware, by no means all experimental data show clear evidence for the existence of time dilation both in...

Take a look at the link provided by chroot. The effect has been extremely well tested in many different settings.

Also, time dilation is closely linked to the mass-energy relation, for which there are tons of evidence, both from nuclear plants and weapons, and from day-to-day results in accelerator laboratories. Everyday, lots of data are stored and analysed that show how two colliding particles of mass 1 can produce many particles of mass close to 170, this extra mass coming from the kinetic energy of the original particles, in precise agreement with the mass-energy relation.

Originally posted by ahrkron ...Also, time dilation is closely linked to the mass-energy relation, for which there are tons of evidence, both from nuclear plants and weapons, and from day-to-day results in accelerator laboratories.

That's true for relativistic mass. But I don't see how this relates to rest mass. What do you mean with respect to the equivalence of rest mass and rest energy?